High impact strength thermoplastic compositions

ABSTRACT

There are provided high impact strength thermoplastic compositions comprising a polyphenylene ether and a rubber modified polystyrene resin, the polystyrene matrix in the composition having an intrinsic viscosity of at least 1.0 deciliters/gram. Such compositions, which contain polystyrene of substantially higher molecular weight than heretofore, provide molded articles with unexpected improvements in impact resistance, surface appearance and resistance to aggressive solvent systems. Also provided are high impact compositions, useful per se or for blending with polyphenylene ethers, comprising polyblends of rubber dispersed in a polystyrene matrix, the polystyrene having an intrinsic viscosity of at least 1.0 deciliters/gram.

This is a division of application Ser. No. 183,630, filed Sept. 24,1971, now U.S. Pat No. 3,819,671.

This invention relates to thermoplastic resin compositions and, moreparticularly, to high impact thermoplastic compositions comprising apolyphenylene ether and a rubber modified polystyrene resin, in whichthe intrinsic viscosity of the polystyrene in the matrix is at leastabout 1.0 deciliters/gram.

BACKGROUND OF THE INVENTION

The polyphenylene ethers are known and described in numerouspublications, including Hay, U.S. Pat. Nos. 3,306,874 and 3,306,875; andStamatoff, U.S. Pat. Nos. 3,257,357 and 3,257,358, all incorporatedherein by reference. They are useful for many commercial applicationsrequiring high temperature resistance and, because they arethermoplastic, they can be formed into films, fibers and moldedarticles. In spite of these desirable properties, parts molded frompolyphenylene ethers are somewhat brittle due to poor impact strength.In addition, the relatively high melt viscosities and softening pointsare considered a disadvantage for many uses. Films and fibers can beformed from polyphenylene ethers on a commercial scale using solutiontechniques, but melt processing is commercially unattractive because ofthe required high temperatures needed to soften the polymer and theproblems associated therewith such as instability and discoloration.Such techniques also require specially designed process equipment tooperate at elevated temperatures. Molded articles can be formed by meltprocessing techniques, but, again, the high temperatures required areundesirable.

In addition, although the polyphenylene ether resins have outstandinghydrolytic stability, making them very useful in contact with aqueousmedia, e.g., in dishwasher and laundry equipment, they will soften ordissolve in contact with many aggressive solvents, e.g., halogenated oraromatic hydrocarbons and gasoline, which limits their use in automotiveapplications.

It is known in the art that the properties of the polyphenylene etherscan be materially altered by forming compositions with other polymers.For example, Finholt, U.S. Pat. No. 3,379,792, discloses that flowproperties of polyphenylene ethers are improved by preparing acomposition thereof with from about 0.1 to 25 parts by weight of apolyamide. In Gowan, U.S. Pat. No. 3,361,851, polyphenylene ethers areformed into compositions with polyolefins to improve impact strength andresistance to aggressive solvents. In Cizek, U.S. Pat. No. 3,383,435,incorporated herein by reference, there is provided a means tosimultaneously improve the melt processability of the polyphenyleneethers and upgrade many properties of polystyrene resins. The Cizekpatent disclosed that polyphenylene ethers and polystyrene resins,including many modified polystyrenes, are combinable in all proportionsto provide compositions having many properties improved over those ofeither of the components.

Preferred embodiments of the Cizek patent are compositions comprising arubber modified high-impact polystyrene and apoly-(2,6-dialkyl-1,4-phenylene)ether. Such compositions are importantcommercially because they provide both an improvement in the meltprocessability of the polyphenylene ether and an improvement in theimpact resistance of parts molded from the compositions. Furthermore,such compositions of the polyphenylene ether and the rubber modifiedhigh-impact polystyrene may be custom formulated to providepre-determined properties ranging between those of the polystyrene resinand those of the polyphenylene ether by controlling the ratio of the twopolymers. The reason for this is that the Cizek compositions exhibit asingle set of thermodynamic properties rather than the two distinct setsof properties i.e., one for each of the components of the composition,as is typical with compositions or blends of the prior art.

The preferred embodiment of the Cizek patent is disclosed to comprisepoly(2,6-dimethyl-1,4-phenylene)ether and a rubber modified high-impactpolystyrene (identified in Example 7 as Lustrex HT88-1 of MonsantoChemical Company). It is known in the art that Monsanto HT-88 highimpact polystyrene contains an elastomeric gel phase dispersed through apolystyrene matrix and that this elastomeric phase comprises about 20.7%by weight of the composition. In addition, it is known that in the gelfree polystyrene matrix in Lustrex 88, the weight average molecularweight, M_(w), is about 251,000 and the number average molecular weight,M_(n), is about 73,000, and, therefore, the polydispersity, i.e., theratio M_(w) /M_(n) is about 3.44. This is shown, for example, in Table 3in Vol. 13, Encyclopedia of Polymer Science and Technology, 1970, p. 401et seq. Thus the preferred embodiment of the Cizek patent, which wasdisclosed to have a notched Izod impact strength ranging from 1.0 to 1.5ft.lbs./in. notch (Standard Method, ASTM- D-256) comprised apolyphenylene ether and a rubber modified high-impact polystyrene resin,the polystyrene in the matrix having a weight average molecular weightof about 251,000.

The Staudinger equation

    [η] = KM.sup.a

wherein [η] is the intrinsic viscosity, K is a constant, M is themolecular weight (close to the weight average) and a is a constantdepending on the system, is used by those skilled in the art todetermine relative molecular weights in a given polymer system. For thepurposes of this disclosure, the relative molecular weights of thematrix polystyrene will be discussed in terms of intrinsic viscosity.The intrinsic viscosity of a polymer solution is usually estimated bydetermining the specific viscosity at several low concentrations andextrapolating the values to zero concentration. Determined in knownways, the intrinsic viscosity of the matrix in Lustrex HT-88 is of theorder of 0.8 deciliters/gram.

It is generally recognized that the properties of impact resistantpolystyrenes are highly dependent on the number, size and character ofdispersed elastomeric particles. Moreover, while most commercial impactpolystyrenes contain from 3 to 10% by weight of a dispersed rubber phasecomprising particles of polybutadiene or rubbery butadiene-styrenecopolymer, the polystyrene in the matrix usually has a limiteddistribution of weight average molecular weights and, especially, theupper limit appears to be about 260,000 (the same as Lustrex-88 used inCizek). By way of illustration, the four commercial products shown inthe Encyclopedia of Polymer Science and Technology, Interscience, Vol.13, p. 400 (1970), Table 3 have M_(w) values of 209,000; 251,000;252,000 and 164,000.

Moreover, as part of a study of the effect of the molecular weightdistribution of the matrix polystyrene in high impact polystyrenes,Wagner et al., Encyclopedia of Rubber Technology, Vol. 43, 1970, p.1136, stated as a generally recognized fact that commercial thermallyinitiated impact polystyrenes have a weight average molecular weight,M_(w), in the range of 250,000. Proceeding from this point, Wagner et alblended in increasing amounts of polystyrene of higher molecular weight,M_(w), 305,000 (intrinsic viscosity of about 0.90). In so doing, therubber content was decreased, and there was found a gradual decrease inproperties, particularly in impact strength.

These combined teachings indicate that increasing the molecular weightof the polystyrene in the matrix of the rubber modified polystyrene usedin the Cizek embodiments would not increase the physical properties ofthe composition with polyphenylene ether resins, but would tend to causethem gradually to decrease.

In view of the above, it has now unexpectedly been found thatcompositions of a polyphenylene ether with a rubber modified polystyreneresin can be provided with substantially improved impact strengths ifthe polystyrene matrix phase has an intrinsic viscosity of at least 1.0deciliters/gram measured in chloroform at 30°C. Correspondingly, theweight average molecular weight, M_(w), is above 350,000, which issubstantially above the 305,000 in the additive used by Wagner et al.,and the range of 164,000-252,000 used in the prior art.

In addition, in such compositions, the surface appearance, especiallygloss, is unexpectedly improved, as is the resistance to aggressivesolvents, such as gasoline.

With respect to gasoline resistance, it is disclosed in the Cizek patentthat this can be improved by using as the polystyrene resin component,in combination with the polyphenylene ether resin a copolymerizedalkenyl cyanide, such as acrylonitrile, either in the rubber backbone orcopolymerized with the styrene. Without such an expedient, the styreneresins used in the Cizek compositions, e.g., Styron-666 and LustrexHT-88, provide molded parts which have poor gasoline resistance,precluding their use in many applications, particularly automotive uses.

It has also now been discovered that the gasoline resistance ofpolyphenylene ether resins and blends thereof with styrene resins isremarkably improved if the molecular weight of polystyrene in the matrixis significantly increased above that in the commercial products andthose shown in the prior art. While the prior art compositions, whichhave polystyrene matrix intrinsic viscosities in the range 0.75-0.90,provide 40/60 and 40/65 compositions withpoly(2,6-dimethyl-1,4-phenylene)ether that fail catastrophically ingasoline at 1% strain in 10-15 seconds, if the intrinsic viscosity isincreased to above 1.0, in accordance with this invention, no failure,is seen even after 30 minutes.

DESCRIPTION OF THE INVENTION

According to the present invention, in its broadest aspects, there areprovided thermoplastic compositions with unexpectedly high impactresistance and resistance to aggressive solvents comprising apolyphenylene ether, and a rubber modified polystyrene, the polystyrenematrix in the composition having an intrinsic viscosity of at leastabout 1.0, and preferably about 1.0 to 1.5 deciliters/gram. Foruniformity, the intrinsic viscosities are to be measured in chloroformat 30°C. Preferably the polystyrene in the matrix has a broad molecularweight distribution, with a polydispersity, i.e., M_(w) /M_(n) ratio ofat least 3.5. in general, the compositions according to this inventionare prepared by combining the polyphenylene ether and a rubber modifiedpolystyrene resin to obtain a composition having at least two phases,one of which is discontinuous and comprises rubber particles. The otherphase is a matrix of polyphenylene ether and polystyrene resin. Suchcompositions may be molded to shape using conventional moldingprocedures. Preferably the rubber is a diene rubber.

Therefore, according to a preferred aspect of this invention, there areprovided high impact strength thermoplastic compositions comprising

a. a polyphenylene ether and

b. a rubber modified polystyrene resin,

the rubber modified polystyrene containing an elastomeric phasedispersed in a polystyrene matrix, the rubber being a polystyrenegrafted diene rubber and the polystyrene in the matrix having anintrinsic viscosity of at least about 1.0. Preferably the rubber contentof the composition is from 4 to 20% and especially preferably, it isfrom about 6 to 12%, based on the resinous components in thecomposition.

Methods to determine the intrinsic viscosity of the polystyrene matrixare well known to those skilled in the art. One convenient methodcomprises separating first any gel phase from the rubber modifiedpolystyrene by high speed centrifugation. In one procedure, a 5% byweight suspension of rubber modified polystyrene is kept in contact witha mixture of methyl ethyl ketone and acetone (50/50 by volume) for 90minutes with mild shaking. Then it is centrifuged at 47,000 × G (19,500r.p.m.) at 10°C. Any gel phase is recovered by decanting and vacuumdrying at 50°C. The polystyrene is recovered by precipitating withmethanol from the centrifugate. Solutions are made up in chloroform andthe intrinsic viscosities are determined at high dilutions at 30°C. bystandard methods. On the other hand, if the rubber is combined bymechanical blending into the styrene resin, the intrinsic viscosity iseasily measured beforehand, and a blending method is used to insure thatthe molecular weight of the polystyrene is not degraded.

As has been mentioned, preferred compositions are made from polystyreneand rubber in which the styrene phase has a polydispersity of greaterthan 3.5 and, especially preferably, from about 3.5 to about 5.0. Theterm polydispersity means the ratio of weight average molecular weightto number average molecular weight -- the higher the ratio, the broaderthe range of molecular weights. Impact resistance, especially, seems tobe highest at a given rubber content when the polydispersity is greaterthan 3.5.

Polydispersity can be determined in ways known to those skilled in theart. For example, the ratio is conveniently measured by gel permeationchromatography.

The compositions of this invention generally consist of a mixture of twophases, the continuous phase being a matrix of polyphenylene oxide resinand styrene resin in which there is a discontinuous rubbery phasedispersed comprising particles of elastomer. Such particles may alsoinclude to varying extents polyphenylene ether resins, depending uponhow the compositions are prepared. If the particles are prepared bygrafting, in general, it is preferred that they include a minorproportion, e.g., up to about 50% by weight of ungrafted polystyrene. Ina typical particle there may be, for example, up to about 45% by weightof rubber, about 10% or more by weight of grafted polystyrene and up toabout 50% by weight of occluded, ungrafted styrene resin. The ungraftedstyrene resin in the gel will have an intrinsic viscosity of the orderof that in the matrix.

The present compositions are most conveniently prepared by combining arubber modified polystyrene resin with the polyphenylene ether. Theparticles of the elastomer are provided, e.g., by polyblending, i.e.,mechanically mixing the components or by chemical interpolymerization,e.g., by polymerizing styrene in the presence of dissolved rubber underwell known conditions whereby a dispersed microgel of, e.g., polystyrenegrafted, cross-linked rubber particles becomes dispersed in apolystyrene matrix. With polyblending, the rubber particles will havelittle or no gel and in such compositions there will be no polystyrenegrafted gel phase -- although in all such composition,interpolymerization between the polyphenylene ether-polystyrene and/orthe rubber can occur. These rubber modified polystyrene resins are thencombined with the polyphenylene ether resins and the amount of rubber inthe final composition is directly related to the amount of rubbermodified polystyrene used. In addition, if blending is carried out,according to conventional procedures, permitting little or nodegradation in molecular weights, the polystyrene matrix will have theintrinsic viscosity corresponding to that which is used to make up thepresent compositions.

The polyphenylene ethers with which this invention is concerned arefully described in the above-mentioned references. The polyphenyleneether are self-condensation products of monohydric monocyclic phenolsproduced by reacting the phenols with oxygen in the presence of complexcopper catalysts. In general, molecular weight will be controlled byreaction time, longer times providing a higher average number ofrepeating units.

A preferred family of polyphenylene ethers will have repeatingstructural units of the formula: ##SPC1##

wherein the oxygen ether atom of one unit is connected to the benzenenucleus of the next adjoining unit, n is a positive integer and is atleast 50, and each Q is a monovalent substituent selected from the groupconsisting of hydrogen, halogen, hydrocarbon radicals free of a tertiaryalpha-carbon atoms, halohydrocarbon radicals having at least two carbonatoms between the halogen atom and the phenyl nucleus, hydrocarbonoxyradicals and halohydrocarbonoxy radicals having at least two carbonatoms between the halogen atom and the phenyl nucleus.

Illustrative members are: poly(2,6-dilauryl-1,4-phenylene) ether;poly(2,6-diphenyl-1,4-phenylene)ether;poly(2,6-dimethoxy-1,4-phenylene)ether;poly(2,6-diathoxy-1,4-phenylene)ether;poly-(2-methoxy-6-ethoxy-1,4-phenylene)ether;poly(2-ethyl-6-stearyloxy-1,4-phenylene)ether;poly(2,6-dichloro-1,4-phenylene)ether;poly-(2-methyl-6-phenyl-1,4-phenylene)ether;poly(2,6-dibenzyl-1,4-phenylene)ether;poly(2-ethoxy-1,4-phenylene)ether; poly(2-chloro-1,4-phenylene)ether;poly(2,5-dibromo-1,4-phenylene)ether; and the like. Examples ofpolyphenylene ethers corresponding to the above formula can be found inthe above referenced patents of Hay and Stamatoff.

For purposes of the present invention an especially preferred family ofpolyphenylene ethers include those having alkyl substitution in the twopositions ortho to the oxygen ether atom, i.e., those of the aboveformula wherein each Q is alkyl, most preferably having from 1 to 4carbon atoms. Illustrative members of this class are:poly(2,6-dimethyl-1,4-phenylene)ether;poly(2,6-diethyl-1,4-phenylene)ether;poly(2-methyl-6-ethyl-1,4-phenylene) ether;poly(2-methyl-6-propyl-1,4-phenylene)ether;poly(2,6-dipropyl-1,4-phenylene)ether;poly(2-ethyl-6-propyl-1,4-phenylene) ether; and the like.

The most preferred polyphenylene ether resin for purposes of the presentinvention is poly(2,6-dimethyl-1,4-phenylene)ether. This resin readilyforms a compatible and single phase composition with polystyrene resinsover the entire range of combining ratios.

In the present compositions, the polyphenylene ether is combined with astyrene resin and a rubber. Suitable resins for the polystyrene matrixare shown in Cizek, U.S. Pat. No. 3,383,435. However, they will all havean intrinsic viscosity higher than usual, i.e., at least 1.0, and willhave a weight average molecular weight of at least 350,000 and,preferably a high polydispersity, i.e., a broad variance betweenmolecular weight fractions, around the 350,000 and above average minimumlevel. Such resins will be combinable with the polyphenylene ether and,in general, will be selected from those having at least 25% by weight ofthe polymer units derived from a vinyl aromatic monomer, e.g., onehaving the formula: ##SPC2##

wherein R is hydrogen, (lower)alkyl, e.g., of from 1 to 4 carbon atomsor halogen; Z is hydrogen, vinyl, halogen or (lower)alkyl; and p is O ora whole number of from 1 to 5. Illustrative polystyrene resins includehomopolymers of polystyrene; polychlorostyrene; poly-α-methylstyrene;and the like, styrene-containing copolymers, such asstyrene-acrylonitrile copolymers; copolymers of ethylvinylbenzene anddivinylbenzene; styrene-acrylonitrile-α-methylstyrene terpolymers; andthe like. Preferred polystyrene resins of this class arehomopolystyrene; poly-α-methylstyrene; styrene-acrylonitrile copolymers;styrene-α-methylstyrene copolymer; styrene-methyl methacrylatecopolymer; and poly-α-chlorostyrene. Especially preferred as the monomerfor preparing the present compositions is styrene monomer.

The "rubber" used to modify the polystyrene resin includes polymericmaterials, natural and synthetic, which are elastomers at roomtemperature, e.g., 20° to 25°C. The term "rubber" includes, therefore,natural or synthetic rubbers of the diene elastomer type generally usedin preparing impact polymers. All such rubbers will form a two phasesystem with the polystyrene resin, and will comprise the discontinuousparticulate rubbery phase in both the impact resistant styrene resin andthe polyphenylene ether-polystyrene-rubber compositions of thisinvention.

Illustrative rubbers for use in this invention are natural rubber andpolymerized diene rubbers, e.g., polybutadiene, polyisoprene, and thelike, and copolymers of such dienes with vinyl monomers, e.g., vinylaromatic monomers, such as styrene. Examples of suitable rubbers orrubbery copolymers are natural crepe rubber, synthetic SBR type rubbercontaining from 40 to 98% by weight of butadiene and from 60 to 2percent by weight of styrene prepared by either hot or cold emulsionpolymerization, synthetic GR-N type rubber containing from 65 to 82percent by weight of butadiene and from 35 to 18 percent by weight ofacrylonitrile, and synthetic rubbers prepared from, for example,butadiene, butadiene-styrene or isoprene by methods, e.g., thoseemploying heterogeneous catalyst systems, such as a trialkylaluminum anda titanium halide. Among the synthetic rubbers which may be used inpreparing the present compositions are elastomeric modified dienehomopolymers, e.g., hydroxy- and carboxy-terminated polybutadienes;poly-chlorobutadienes, e.g., neoprenes; copolymers of dienes, e.g.,butadiene and isoprene, with various comonomers, such as alkylunsaturated esters, e.g., methyl methacrylate; unsaturated ketones,e.g., methylisopropenyl ketone, vinyl heterocyclics, e.g., vinylpyridine; and the like. The preferred rubbers comprise polybutadiene andrubbery copolymers of butadiene with styrene. Such preferred rubbers arewidely used in forming rubber modified high impact polystyrene resinswith the range of matrix polystyrene intrinsic viscosities and molecularweights mentioned in the above-cited references.

The term "rubber modified polystyrene resin" defines a class ofcompounds comprising a two-phase system in which rubber is dispersed ina polystyrene resin matrix in the form of discrete particles. Theparticles can be formed by a mechanical blending of the rubber and thepolystyrene resin and, if a cross-linking agent, e.g., a sulfur ispresent, the particles will comprise a dispersed gelled elastomericphase. On the other hand, the two-phase system will consist ofinterpolymers of a styrene monomer and an elastomer or rubber.Commercially, such high impact polystyrenes are usually made by graftingof rubber in the presence of polymerizing styrene. Such systems consistof a continuous phase of the polymerized styrene monomer in which therubber or elastomer is dispersed in a discontinuous elastomeric gelphase, in some cases without, but in most cases, and preferably, withgrafted chains of polymerized vinyl aromatic, e.g., styrene, monomer.The particles will usually contain occluded, polymerized styrenemonomer, too, and this can comprise up to about 50% of their weight,exclusive of grafted polystyrene.

Methods for the production of rubber modified polystyrenes for use inthis invention, with the high molecular weight polystyrene matrix willbe known to those skilled in the art. In general, if the particles ofrubber are dispersed mechanically by blending rubber with polystyrene, avery high molecular weight crystal polystyrene will be used. These canbe made by common techniques, e.g., bulk or suspension polymerization,the high molecular weight being achieved by the expedients of using verylittle or no catalyst, a lower than normal polymerization temperature,very little or no chain transfer agent, a substantially increasedpolymerization time, or a combination of any of the foregoing all ofwhich are known to increase the molecular weight of the polystyrene

One useful method, which will be exemplified hereinafter, provides apolystyrene with an intrinsic viscosity of a weight average molecularweight of 479,000, a number average molecular weight of 129,000,respectively.

Although such high molecular weight polystyrenes can be blended withrubber directly, it is more convenient, and preferred first to polyblendthe rubber with a lower molecular weight polystyrene, e.g., intrinsicviscosity about 0.7-0.9 and M_(w), 115,000-300,000 and then to polyblendthis with the high molecular weight polystyrene, to obtain any desiredrubber content and a matrix intrinsic viscosity of at least 1.0,preferably from about 1.0 to 1.5 deciliters/gram, measured in chloroformat 30°C. Such compositions have high impact strength and excellentresistance to aggressive solvents per se and in combination withpolyphenylene ether resins, and are contemplated by this invention. Itis preferred that the rubber be a diene rubber, as hereinabove defined,and that it be present in an amount to provide from 4 to 20% of thetotal weight of the styrene resin plus the rubber in the composition.

Methods for preparing rubber modified polystyrenes with dispersedstyrene grafted elastomeric particles are well known. The molecularweight in the polystyrene matrix can be increased by varying thepolymerization conditions, e.g., by decreasing or eliminating catalyst,decreasing or eliminating chain transfer agent, lowering thetemperature, increasing the polymerization time, blending in very highmolecular weight styrene resin, as mentioned above. These expedients canbe used in well known processes, such as that of Amos. et al., U.S. Pat.No. 2,694,692, in which polymerization of rubber in styrene monomer iscarried out in bulk and the mixture is agitated during the beginningstages to form the desired amount of rubber particles and then stirringis reduced and polymerization is completed. Also useful is the methoddescribed in Stein et al., U.S. Pat. No. 2,886,553, in which a bulkpre-polymerization of rubber in styrene monomer is carried out withheating, agitating until the desired rubber particle content is obtainedthen water and surfactants are added and polymerization is completed insuspension. The rate of agitation in the prepolymerization step in bothprocesses controls the grafted particle content. Such procedures withattention to the process variables mentioned above can readily providerubber modified polystyrenes with intrinsic viscosities above 1.0. Suchcompositions, which are grafted products, in contrast to the polyblendsmentioned above, are also commercially available from Koppers Companyunder product designation PRX-1004 and PRX-1005, intrinsic viscosities1.07 and 1.22, respectively.

As is described in Cizek, U.S. Pat. No. 3,383,435, polyphenylene ethersand polystyrene resins are combinable with each other in all proportionsand they exhibit a single set of thermodynamic properties. The presentcompositions therefore can comprise from 1 to 99% by weightpolyphenylene ether resin and from 99 to 1% polystyrene resin, on arubber-free basis, and these are included within the scope of theinvention. In general, compositions in which the polystyrene resin, on arubber-free basis, comprises from 20 to 80% by weight of the polystyreneand the polyphenylene ether, are preferred because after molding theyhave the best combination of impact strength, surface appearance andresistance to solvents. Particularly useful and preferred arecompositions in which the polystyrene resin, on a rubber-free basis,comprises from 40 to 60% by weight of the combined weight of thepolystyrene and the polyphenylene ethers. Properties, such as flexuralstrength, tensile strength, hardness and especially impact strengthappear to be at a maximum in such preferred compositions.

The rubber phase, i.e., the weight percentage of the dispersedelastomeric phase, in the instant compositions can vary, although noadvantage is secured in exceeding a maximum of about 30% by weight ofthe total weight of the composition. If the elastomeric phase contentfalls below about 0.1% by weight, impact properties decline. Thepreferred range of elastomeric gel phase content is from about 4 toabout 20% by weight, with the higher amount being used when the rubberif dispersed by polyblending, i.e., mechanical blending. If, as ispreferred, all of the rubber is in the form of an elastomericpolystyrene grafted diene rubber, the lower amounts can be advantageous.In all cases, the preferred amount of elastomeric phase will rangebetween 6 and 12% by the total weight of the composition. Although, athigher levels, impact strength is clearly optimized, other properties,such as solvent resistance and appearance of molded parts are affected.Because the grafted rubber particles provide compositions with betterimpact strengths than those from mechanically blended, i.e., ungraftedbut gelled, particles at the optimum level, the compositions of thisinvention containing particulate styrene grafted elastomer phase areespecially preferred.

The method used to form the polyphenylene ether-polystyrene-rubbercompositions of the invention is not critical provided that it permitsefficient dispersion and mixing. The preferred method is one in whichthe polyphenylene ether is mixed with a polystyrene and a rubber orrubber modified polystyrene using any conventional mixing method and thecomposition so formed is molded to any desired shape such as byextrusion, hot molding, injection molding, and the like.

It should, of course, be obvious to those skilled in the art that theother additives may be included in the present compositions such asplasticizers, pigments, flame retardant additives, reinforcing agents,such as glass filaments or fibers, stabilizers and the like.

The following procedure illustrates the preparation of a very highmolecular weight crystal polystyrene which is useful for formingpolyblends with rubber, and ultimately for incorporation into thepolyphenylene ether-polystyrene-rubber compositions of this invention.

PROCEDURE

A flask fitted with a stirrer, condenser and nitrogen flushing system ischarged with 2000 ml. of distilled water, 50 g. of styrene monomer(washed free of inhibitor with dilute caustic solution), 2.0 g. ofbenzoyl peroxide initiator and 5 g. of gelatin. Polymerization iscarried out in stirred suspension at 70°C. for 24 hours. The beads ofpolystyrene are removed by filtration, washed with water and vacuumdried. The yield is 92% of the theoretical, based on styrene monomer.

The product has weight and number average molecular weights of 479,000and 129,000, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will be further illustrated by the following exampleswherein, unless otherwise indicated, all compositions are prepared bypassing mixtures of the polyphenylene ether, the styrene resin and therubber or the high-impact polystyrene and other ingredients, if present,through a variable pitch, single screw extruder with extrusiontemperature maintained between about 450° and 600°F. All parts are byweight. The strands emerging from the extruder are cooled, chopped intopellets and molded into test bars using standard procedures. Izod impactstrengths are determined on 1/8 inch thick specimens by ASTM D-256-56.Resistance to aggressive solvents is determined by placing 1/8 inch testpiece in a jig under a 1% flexural strain and immersing it in gasoline.The time is noted for crazing or failure.

EXAMPLE 1

The following formulation is blended:

    Material                   Parts                                              ______________________________________                                        Poly(2,6-dimethyl-1,4-phenylene)ether*                                                                   40                                                 Rubber modified polystyrene**                                                                            65                                                 ______________________________________                                         *General Electric Company, PPO polyphenylene ether, in pellet form.           **Koppers Company, PRX-1004, high impact rubber modified polystyrene, in      pellet form, containing about 7.5% by weight of polybutadiene in the form     of a polystyrene grafted elastomeric phase dispersed in a matrix of           polystyrene. The molecular weight of polystyrene in the matrix is             determined by dissolving out the soluble fraction with 1:1 volume of          methyl ethyl ketone and acetone, centrifuging to separate the insolubles,     and precipitating the polystyrene from the centrifugate with methanol. Th     intrinsic viscosity is 1.07 deciliters/gram, measured in chloroform at        30°C., the number average molecular weight of the polystyrene in       the matrix is about 96,800 and the weight average molecular weight is         about 382,000 (M.sub.w /M.sub.n = 3.95).                                 

The mixture is extruded in a 3/4 inch Wayne extruder. The resultantstrands are cooled, chopped into pellets and molded into test specimens.

The following physical properties are obtained:

    Izod impact (ft.lbs./in. notch)                                                                     3.6                                                     Elongation to failure (%)                                                                           59                                                      Tensile yield strength (psi)                                                                        9600                                                    Tensile ultimate strength (psi)                                                                     7400                                                

The gasoline resistance is determined by immersion at 1% flexuralstrain. There is no crazing or failure after 30 minutes, when the testis terminated.

EXAMPLE 2

The following formulation is blended:

    Material             Parts by weight                                          ______________________________________                                        Poly(2,6-dimethyl-1,4-phenylene)                                              ether (as in Example 1)                                                                            40                                                       Rubber modified polystyrene*                                                                       65                                                       ______________________________________                                         *Koppers Company, PRX-1005, high impact rubber modified polystyrene, in       pellet form, containing about 7.5% by weight of polybutadiene in the form     of a polystyrene grafted elastomeric phase dispersed in a matrix of           polystyrene. The intrinsic viscosity of the polystyrene matrix is 1.22;       M.sub.w is 489,000; M.sub.n is 80,200 and M.sub.w /M.sub.n is 6.1.       

The following physical properties are obtained:

    Izod impact (ft.lbs./in. notch)                                                                     3.82                                                    Elongation to failure (%)                                                                           47                                                      Tensile yield strength (psi)                                                                        9100                                                    Tensile ultimate strength (psi)                                                                     7400                                                

The gasoline resistance is measured at 1% flexural strain. There is nocrazing or failure after 30 minutes, when the test is terminated.

COMPARATIVE EXAMPLES A-C

For purposes of comparison, the procedure of Example 1 is repeatedsubstituting for the rubber modified high-impact polystyrene having apolystyrene matrix with intrinsic viscosity of 1.07, three commerciallyavailable rubber modified high impact polystyrenes having polystyreneintrinsic viscosities in the conventional range of about 0.8-0.9.

The following formulations are blended:

                        Parts by weight                                           Material              A       B       C                                       ______________________________________                                        Poly(2,6-dimethyl-1,4-phenylene)                                              ether (as in Example 1)                                                                             40      40      40                                      Rubber modified polystyrene                                                   (matrix [η], 0.87)*                                                                             65                                                      Rubber modified polystyrene                                                   (matrix [η], 0.79)**      65                                              Rubber modified polystyrene                                                   (matrix [η], 0.80)***             60                                      ______________________________________                                         *Koppers Company, Dylene 601, in pellet form, containing about 80% by         weight of polybutadiene in the form of a polystyrene grafted elastomeric      phase dispersed in a matrix of polystyrene. The intrinsic viscosity of th     polystyrene matrix is 0.87.                                                   **Cosden Chemical Corp., 825 TV, in pellet form containing about 8% by        weight of polybutadiene in the form of a polystyrene grafted elastomeric      phase dispersed in a matrix of polystyrene. The intrinsic viscosity of th     polystyrene matrix is 0.79, M.sub.w 276,000; M.sub.n is 68,700, M.sub.w       /M.sub.n is 4.02.                                                             ***Monsanto Company, HT-91, in pellet form containing about 8% by weight      of polybutadiene grafted elastomeric phase dispersed in a matrix of           polystyrene. The intrinsic viscosity of the polystyrene matrix is 0.80,       M.sub.w is 240,000; M.sub.n is 86,000 and M.sub.w /M.sub.n is 2.79.      

The following physical properties are obtained:

                     A      B      C                                              __________________________________________________________________________    Izod impact (ft.lbs./in. notch)                                                                1.84   1.78   1.76                                           Elongation to failure (%)                                                                      46     19     30                                             Tensile yield strength (psi)                                                                   9000   9100   8400                                           Tensile ultimate strength (psi)                                                                7600   7400   7800                                           __________________________________________________________________________

In the gasoline immersion test at 1% strain, Sample A catastrophicallyfailed in about 15 seconds, Sample B in less than 15 seconds and SampleC in less than 10 seconds.

A comparison of the results of Examples 1 and 2 with those of Samples A,B and C demonstrates that at approximately equal rubber contents andwith about the same amount of polyphenylene ether, there has beenobtained a substantial improvement in impact strength as measured in theIzod tests in the compositions which are prepared from the polystyrenehaving an intrinsic viscosity greater than 1.0. In addition there is anoutstanding increase in resistance to gasoline when the molecular weightof the polystyrene is substantially above that found in the commercialproducts.

EXAMPLE 3

A high impact polystyrene composition with superior physical propertiesand solvent resistance is prepared by polyblending (using coextrusion):

    Material              Parts by weight                                         ______________________________________                                        Rubber modified polystyrene*                                                                        33                                                      Crystal polystyrene ([η]. 1.20)**                                                               67                                                      ______________________________________                                         *Union Carbide Co., TGD-2100, polystyrene containing 24% by weight of         polybutadiene rubber.                                                         **Crystal polystyrene, intrinsic viscosity of about 1.20, M.sub.w,            479,000; M.sub.n, 129,000; M.sub.w /M.sub.n, 3.7.                        

This material, in which the matrix polystyrene has an intrinsicviscosity of greater than 1.0, is molded and the properties are comparedwith those of a commercial high impact polystyrene (Monsanto Company,HT-91) which also has about 8% by weight of rubber, but in the form ofgrafted particles. The intrinsic viscosity and weight and number averagemolecular weight of the polystyrene matrix in HT-91 are 0.80 dl./g.;240,000 and 86,000, respectively.

The following physical properties are obtained:

                  Polyblend     Commercial                                                      of this       Graft                                             Properties    invention     polymer                                           ______________________________________                                        Izod impact strength                                                                        2.6           1.5                                               (ft.lbs./in. notch)                                                           Tensile yield strength (psi)                                                                5200          6600                                              Elongation at failure, %                                                                    42            26                                                Flexural modulus (psi)                                                                      344000        312000                                            ______________________________________                                    

At 1% flexural strain in gasoline the commercial graft polymer failedcatastrophically, whereas the polyblend according tok this invention didnot show any cracks or crazes in 10 minutes, although the surface wasslightly tacky.

The above properties demonstrate that increasing the molecular weight ofthe polystyrene and polyblending provides superior impact strength,elongation and flexural modulus. The composition also has substantiallyincreased resistance to attack by gasoline.

EXAMPLE 4

The following formulation is blended:

    Materials             Parts by weight                                         ______________________________________                                        Poly(2,6-dimethyl-1,4-phenylene)                                              ether (as in Example 1)                                                                             40                                                      Rubber modified polystyrene (Union                                            Carbide (TGD-2100, as in Example 3)                                                                 20                                                      Crystal polystyrene ([η], 1.20 as                                         in Example 3)         40                                                      ______________________________________                                    

The composition has higher impact strengths than correspondingcompositions in which the polystyrene in the matrix has an intrinsicviscosity of below about 1.0 and excellent resistance to gasoline.

EXAMPLE 5

The procedure of Example 1 is repeated, substituting for the polystyrenemodified with polybutadiene, a polystyrene containing 9% by weight ofrubber derived from a rubbery styrene butadiene copolymer containing 77%of butadiene units and 23% of styrene units, by weight. The intrinsicviscosity of the polystyrene matrix is greater than 1.0. The impactstrength of the composition is high.

EXAMPLE 6

The following polyphenylene ethers are substituted forpoly(2,6-dimethyl-1,4-phenylene)ether in the formulation of Example 1:

poly(2,6-diethyl-1,4-phenylene)ether;

poly(2-methyl-6-ethyl-1,4-phenylene)ether;

poly(2-methyl-6-propyl-1,4-phenylene)ether;

poly(2,6-dipropyl-1,4-phenylene)ether

poly(2-ethyl-6-propyl-1,4-phenylene)ether.

Compositions according to this invention are obtained.

Obviously, other modifications and variations of the present inventionare possible in the light of the above teachings. It is, therefore, tobe understood that changes may be made in the particular embodiments ofthe invention described which are within the full intended scope of theinvention as defined by the appended claims.

I claim:
 1. A high impact strength thermoplastic composition consistingessentially of a polyblend of ungrafted particulate rubber dispersed ina polystyrene homopolymer matrix, the polystyrene in said matrix havingan intrinsic viscosity of at least about 1.0 deciliter/gram, measured inchloroform at about 30°C, and wherein said rubber comprises from 4 to20% of the total weight of the composition.
 2. A composition as definedin claim 1 wherein the polystyrene matrix has an intrinsic viscosity inthe range of from 1.0 to 1.5 deciliters/gram.
 3. A composition asdefined in claim 1 wherein said rubber is a diene rubber.
 4. Acomposition as defined in claim 1 wherein said ungrafted rubber isselected from the group consisting of polybutadiene and a copolymer ofbutadiene with styrene.